1. Field of the Invention
This invention relates to a combustion turbine vane assembly, and more specifically, to a combustion turbine vane assembly having endcaps for directing the flow of a coolant, and an associated method of manufacture and assembly of the vane assembly.
2. Background Information
Combustion turbine, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor assembly compresses ambient air. The compressed air is channeled into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is typically at a temperature of between 2500 to 2900xc2x0 F. (1371 to 1593xc2x0 C). The working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vane assemblies and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades to rotate creating a rotation in the shaft.
Typically, the turbine assembly provides a means of cooling the vane assemblies. The first row of vane assemblies, which typically precedes the first row of blades in the turbine assembly, is subject to the highest temperature of working gas. To cool the first row of vane assemblies, a coolant, such as steam or compressed air, is passed through passageways formed within the vane structure. These passageways often include an opening along the trailing edge of the vane to allow the coolant to join the working gas. Such an xe2x80x9copen loopxe2x80x9d system has the disadvantage of reducing the energy of the working gas available to do useful work.
xe2x80x9cClosed loopxe2x80x9d systems allow a coolant to flow through the vane, cooling the vane and absorbing heat, and returning the coolant to be used elsewhere. For example, when the coolant is steam, cool steam is supplied to the vane assemblies and the heated steam may be directed to a steam turbine assembly which is coupled to the closed loop.
An effective closed loop vane cooling design uses a plurality of cooling passages. Prior art closed loop vane assemblies use complicated castings to form the passages. These complex castings, however, have resulted in low manufacturing yields. That is, there is a high rejection rate of the castings during the manufacturing process. The complex casting also required a complex manufacturing process to assemble each vane assembly.
There is, therefore, a need for a turbine vane assembly structured to have a closed loop cooling system which does not require a complicated casting.
There is a further need for a turbine vane structured to have a closed loop cooling system which is easy to assemble.
There is a further need for a turbine vane structure to have a closed loop cooling system which may be easily manufactured.
These needs, and others, are satisfied by the invention which provides a turbine vane assembly having an outer endcap, an inner endcap, and a vane casting with an outer shroud, an inner shroud, and an airfoil. The outer endcap, the inner endcap, the outer shroud, the inner shroud, and the airfoil, which may be jointly called xe2x80x9cthe components,xe2x80x9d each have a plurality of generally straight passageways therethrough which are structured to carry a cooling fluid in a closed loop. Because the passages are generally straight, the passages may be drilled in the components after casting Because the generally straight passageways can be drilled, for example by electro-discharge machining (xe2x80x9cEDMxe2x80x9d) or electrochemical machining (xe2x80x9cECMxe2x80x9d), the vane assemblies and endcaps do not require a complicated casting.
The outer endcap casting includes an inlet port, for allowing a coolant to enter the vane assembly, and an exhaust port, which allows the heated coolant to be routed from the vane assembly to perform useful work elsewhere. The inner endcap casting includes a coolant inlet port. The vane casting includes an integral outer shroud, airfoil, an inner shroud. The outer shroud and inner shroud are structured to be mated with the outer endcap and the inner endcap respectively. When mated, both the inner endcap and the outer endcap form a plurality of plenums with their respective shrouds. The airfoil is essentially hollow, having a main coolant passage and at least one exhaust passage. The plurality of generally straight passages structured to cool the vane assembly are in fluid communication with the outer endcap coolant inlet port and exhaust port. The generally straight passageways within the components are structured to cooperate with the plurality of plenums to create a closed loop cooling system.
Manufacture and assembly of the vane assemblies begins with the casting of the end caps and vane casting. The outer endcap is cast with a coolant inlet port and an exhaust port. The vane casting includes a generally hollow airfoil having a main coolant passage and at least one main exhaust passage. A plurality of openings are machined into the vane casting outer shroud and inner shroud using EDM or ECM. Plugs, machined using wire EDM, are made to partially fill the openings. The cooling passages are then drilled in the components using EDM or ECM. The plugs are inserted into the shroud openings forming the shroud edge plenums. A bond coat is then applied to the components. The bond coat resists oxidation and acts as a bonding layer for the thermal barrier coating which is applied subsequently. The mating surfaces between the endcaps and the vane casting are then machined using conventional machining techniques. Additionally, other features, such as seal slots, end face joints, and shroud hook grooves, may be machined on the components. The endcaps are then heated and fitted into the shrouds forming an interference fit. The interference fit seals the plenums from each other. The joining of the endcaps to the shrouds creates a plurality of plenums in both the outer shroud and the inner shroud. The vane assembly then has a thermal barrier coating applied. Preferably, the airfoil has the coating applied first, then the shrouds and endcaps. Next, an internal aluminized steam corrosion protection is applied.